In recent years, 100 Gigabit-per-second (Gbs) long-distance optical transmission has been implemented by dual-polarization quadrature phase-shift keying (DP-QPSK) using a digital coherent technology. To further improve transmission capacity, greater-level modulation schemes such as polarization division multiplexed 16 quadrature amplitude modulation (16-QAM) are being developed. Nyquist wavelength division multiplexing (WDM) is also being developed to increase the transmission rate.
For optical modulation, Mach-Zehnder optical modulators are generally used, in which modulators a Mach-Zehnder interferometer is formed by a pair of optical waveguide to modulate the optical phase of a light beam. Mach-Zehnder optical modulators have less wavelength (or frequency) chirping compared with a phase modulation scheme using a single waveguide, and are suitable to long-distance optical transmission.
Typically, Lithium niobate (LiNbO3) Mach-Zehnder interferometers making use of the electro-optic effect (Pockels effect) of a dielectric material are used. To reduce a device size, multilevel modulation schemes using a semiconductor Mach-Zehnder interferometer are being developed.
There is an intrinsic problem in semiconductor optical modulators that the modulation characteristic (i.e., the relationship between applied voltage and optical phase change) varies depending on the wavelength of a light beam input to the modulator. In semiconductor optical modulators, the absorption edge wavelength of the semiconductor material changes according to applied voltage, and the phase of light is modulated making use of the phase shift due to absorption based on Kramers-Kronig relations. Hence, semiconductor optical modulators have wavelength dependency such that the closer to the absorption-edge-wavelength the light to be modulated is, the greater the phase change with respect to the voltage change.
To address the wavelength dependency of the modulation characteristic of semiconductor optical modulators, several techniques for controlling a substrate bias voltage or amplitude of a drive signal according to the wavelength of input light are proposed. The first technique is to set the substrate bias voltage to a predetermined level according to the wavelength, and drive the modulator at a constant amplitude of a drive signal regardless of the wavelength. See, for example, Japanese Laid-open Patent Publication No. 2005-326548 A.
The second technique is to perform feedback control on the substrate bias voltage or drive signal amplitude. A low frequency signal is superimposed on driving data signals, and output light signals are monitored. Responsive to the monitoring result, the substrate bias voltage and/or the amplitude of the modulator drive signal is controlled. See, for example, Japanese Laid-open patent publication No. 2012-257164 A.
With the first technique described above, the substrate bias voltage is set to a fixed level according to the wavelength. Accordingly, the modulation characteristic is likely to deviate from the optimum condition. Such deviation occurs when the amplitude of a drive signal output from a driving circuit is influenced by a temperature change or other changes, or when a modulation characteristic itself changes with time. Besides, this method needs to obtain data of the wavelength-to-modulation characteristic of the optical modulator in advance, which takes extra time for testing and is costly.
The second technique uses a driving circuit to control the optical modulator. In order to incorporate the control mechanism for the optical modulator into a product, the driving circuit needs to be built in together with the optical modulator. There are variety of product forms including a module with only a semiconductor optical modulator, or a module in which a semiconductor optical modulator and a semiconductor laser are mounted together. A driving mechanism for an optical modulator uses many driving circuits depending on the modulation scheme (or the modulation rate). For the configuration of controlling the characteristics of an optical modulator using multiple driving circuits, it is difficult to maintain the module size small.
It is desired for fiber optic communication systems to control the modulation characteristics of an optical modulator to be in the optimal conditions, while maintaining the device size small.